Complex segregation analysis was conducted in a sample of 171 pedigrees collected through probands affected by non-syndromic dysplasia of the hip (CDH) treated in Ferrara's CDH Centre in the period 1991-6. The analysis favoured a two-locus model, in which the accepted segregation model at the major locus was compatible with recessive transmission, with a gene frequency of the deleterious allele of around 0.20. For the other locus, among the Mendelian hypotheses tested, the recessive model turned out to be the most parsimonious. When ultrasonographic level was examined as an indication of severity, a significant improvement in the fit of the model was observed, giving clear evidence that information on the severity of CDH is important for a better definition of the hereditary transmission model.